The present invention relates to a method for regenerating a mandrel made of a thermally expandable fluororesin. More particularly, the present invention is concerned with a method for regenerating a mandrel made of a thermally expandable fluororesin which has been used in hollow molding of fiber reinforced plastics (hereinafter referred to as "FRP") comprising a thermoplastic resin as a matrix.
One of the processes for molding a hollow FRP product comprises placing a hollow preform made of an FRP molding material in an outer die, inserting a thermally expandable mandrel into the hollow part of the preform, heating the whole to cause thermal expansion of the mandrel, thereby clamping the preform between the mandrel and the outer die, rapidly cooling the whole to solidify the preform thus treated and shrink the mandrel, withdrawing the shrunk mandrel.
The thermally expandable mandrel for use in this process is required to be excellent not only in the capability of thermal expansion but also in the property of sufficiently clamping the preform of an FRP molding material while retaining the elasticity in the above-described heating-cooling cycle (hereinafter referred to as the elasticity retentiveness) and the property of retaining the similitude of the shape during the expansion-shrinkage cycle in accordance with the heating-cooling cycle (hereinafter referred to as the shape retentiveness).
A fluororesin such as polytetrafluoroethylene resin has hitherto been employed as the raw material of the thermally expandable mandrel. However, the mandrel made of a fluororesin has had a drawback that it suffers from a phenomenon known as the stress relaxation or creep when heated under highly deformed conditions for a prolonged period of time to lower the elasticity retentiveness in the heating step, thereby failing in sufficiently clamping the preform of an FRP molding material inside the outer die and giving the molded product various defects such as deformation, voids and delamination. Further, there has been a problem such that the rapid cooling from the heated state causes even the mandrel which has undergone elastic deformation to have been cooled before it can regain its initial shape, thereby lowering the shape retentiveness. Therefore, the mandrel made of a fluororesin can be reused at most twice or thrice and a further reuse thereof has been almost impossible.